Metabolism-inhibition tests with the porcine mycoplasma strains Mycoplasma hyorhinis, M. granularum and mycoplasma B3 showed that two of these strains, M. hyorhinis and M. granularum, produced colonies in the presence of type-specific rabbit antisera. This type of reaction is referred to as colonial growth of mycoplasma in fluid medium. The colonies were never found in the presence of heterotypic or negative rabbit sera. M. hyorhinis strains showed quantitative and quantitative differences in colonial growth. They produced colonies in the presence of pig sera obtained from herds with a known incidence of M. hyorhinis whereas M. granularum failed to do so in the presence of these sera. Neither of the strains produced colonies in the presence of pig sera obtained from herds from which M. hyorhinis could not be recovered.